docs(architecture): make diagrams explain useful relationships #937

The architecture reference added by #897 contains 42 diagrams, but several repeat adjacent tables or list classes without explaining a useful relationship. Unlabeled arrows, inconsistent color meaning, crowded layouts and very small inline labels make some views harder to use than the prose.

Improve the existing reference so a new maintainer can use each diagram to answer a concrete architecture question.

Acceptance criteria:

• Audit every diagram: retain, redesign, combine or replace with existing prose/tables, recording the reason.
• Keep a substantial opening application map with clear grouping and a visible reading order.
• Give retained views an explicit question, clear scope and labeled relationships; preserve source-backed behavior and important failure cases.
• Use a restrained palette with consistent meaning, text/shape cues beyond color and checked contrast.
• Validate readability at ordinary Markdown width, standalone SVG rendering, editable-source agreement and links.
• Document concise maintenance principles with primary-source design references.

Research basis: C4 diagram/notation guidance, Nielsen Norman Group visual-design principles, and W3C color/contrast guidance.

This is documentation and rendering-tool work; application behavior remains outside scope. Related: #100 and #897.